When I was trying to think of non forecast-related topics to blog about the other day, all the topics that came to my mind were depressing. I didn’t want to write about our worsening drought situation or the the high likelihood of an above-average fire season, and UW professor/blogger Cliff Mass had already written a blog about a ‘hot topic’ in the weather community: the potential decrease in forecasting accuracy due to a lack of aircraft observations from fewer flights during the coronavirus. The long-range (7-16 day) forecasts actually have decreased in skill since the coronavirus outbreak, but correlation doesn’t equal causation, and we don’t know at this time if or how much the lack of observations is affecting forecasting. Still, the decrease in observations can’t be a good thing for forecasting, and seeing as aircraft observations are the 4th most valuable source of data for weather models, they aren’t statistically insignificant. If you’d like to read Professor Mass’ blog, you can find it here.
Then it dawned upon me; how about I write about the decrease in air pollution due to reduced travel from various ‘stay at home’ orders in response to the coronavirus? Well actually, it dawned upon my mom and she passed on the idea to me – I wish I shared her innate optimism! But it seemed like an interesting topic to delve into, and certainly a more optimistic one than the increased risk of forest fires this summer. Maybe I’ll have a forest fire blog after some positive headlines, just to balance everything out.
However, air pollution is not simply a factor of the pollutants emitted into the air; it is also related to the weather patterns that an area experiences. Some of the most misleading articles I’ve seen (and lord knows there are a lot of misleading articles on the internet) compare Beijing’s pollution to the air quality over Seattle or Portland during our summer forest fire episodes while failing to note the clear seasonal aspect to Beijing’s air quality and the fact that late summer autumn tend to be their cleanest month. In the case of Beijing, wintertime heating from coal-fired home stoves plays a significant role, but an even bigger role is played by Beijing’s proclivity for wintertime inversions trapping pollutants near the surface and keeping the city enshrouded in its own noxious fumes.
A particular type of smog – photochemical smog – is formed when ultraviolet radiation and heat reacts with volatile organic compounds (VOCs) and nitrogen oxides to form ozone, peroxylacyl nitrates (PAN), aldehydes. The graph below shows monthly ozone concentrations for 5 cities around the world and was taken from Cooper et al (2014),
Credit: Cooper et al. 2014
Los Angeles sees a ton of photochemical smog in the summer due the large amount of nitrogen oxides and VOCs released from transportation during morning rush hour, the fact that it is in a basin often capped by an inversion, and the intense summer UV radiation it receives. The amount of ozone increases in the atmosphere during the day due to photochemical reactions and subsides at night as the sun sets and ozone reacts with nitrogen oxides and breaks down into simple dioxygen (O2),
Let’s take a look at air pollution across Seattle, which is the location most affected by the coronavirus in the Pacific NW from a transportation and economical standpoint. Governor Inslee’s “Stay Home, Stay Healthy” order was issued on 3/23, but statewide traffic began a long-term downtrend during the first week of March. Traffic bottomed out around 3/28 and has been very slowly rising over the past two weeks. I searched for a database of daily Seattle traffic info but was unable to find any, so this image is taken from an awesome WSDOT blog post written by Bart Treece.
Credit: WSDOT/Bart Treece
The pollutant around the world with the most dramatic drop in levels due to the coronavirus has been nitrogen dioxide (NO2). NO2 is formed when fuel is combusted at high temperatures, as the high temperatures allow oxygen, a highly reactive gas, to bond to nitrogen, a relatively inert gas. NASA’s Aura satellite measures various pollutants, including NO2, and NASA has created some really cool graphs and images showing how NO2 concentrations have changed with the social distancing/stay at home orders from the coronavirus pandemic.
Interestingly, in Seattle, NO2 concentrations only decreased after March 28/29 right when statewide traffic was at a minimum, even though travel had been declining the entire month. With traffic volumes declining, it seems obvious that emissions must have been declining, but I unfortunately could not find daily emissions data.
Credit: NASA
But perhaps meteorological conditions are to blame for the relatively steady concentrations of satellite-measured NO2 through 3/28. The amount of particulate matter in the atmosphere is more related to weather than gases like NO2, as these particulates arise from a variety of both natural and anthropogenic sources. This is particularly true in locations like Seattle that have relatively light wood-burning-stove usage and do not utilize coal-fired power plants.
Credit: Puget Sound Clean Air Agency
Traffic declines from 3/1 – 3/15 were pretty slight and both NO2 and PM2.5 levels remained relatively constant during this period. PM2.5 levels substantially increased from 3/16-22 while traffic declined at a faster rate, and NO2 levels remained relatively constant, perhaps due to unfavorable weather conditions and decreasing emissions offsetting each other. The air cleared up substantially on 3/23 as an upper-level trough passed through and brought cool, northwesterly flow into the region and has remained relatively constant since then. This lines up with NO2 levels being flat through 3/27 (remember, the NO2 graph shows a 13-day rolling average) before dropping precipitously on 3/28. But even after the drop, the change from average has been slight, with NO2 concentrations only about 10-20% less than usual.
In plain English, the decrease in traffic has decreased the amount of NO2 over Seattle, but the decrease has been relatively slight and was only realized after a shift in the weather pattern.
The decrease in NO2 has been much more dramatic for other cities. As mentioned above, Los Angeles is the poster child for emissions of various nitrogen oxides (NOx) and the photochemical smog that develops from it. They were 42% below their 2015-2019 average NO2 levels as of their most recent measurement.
At Wuhan, the center of the outbreak, declines were even more stark, with nitrogen oxide levels reaching nearly 1/3rd of the 2015-2019 average during the lockdown. Restrictions on travel have eased since, and levels are now only 10-20% below average.
With all the hardship and suffering caused by the coronavirus, finding silver linings can take quite a bit of work. But hopefully we can rejoice in knowing that the air we breathe is cleaner in a socially-distant world, even if only a little bit.